domingo, 28 de octubre de 2012

"Hubble's Constant" --The Cosmic Distance Ladder

Astronomers using NASAs Spitzer Space Telescope have greatly improved the cosmic distance ladder used to measure the expansion rate of the universe, as well as its size and age. The cosmic distance ladder, symbolically shown here in this artist's concept, is a series of stars and other objects within galaxies that have known distances. By combining these distance measurements with the speeds at which objects are moving away from us, scientists can calculate the expansion rate of the universe, also known as Hubble's Constant.

Spitzer was able to improve upon past measurements ofHubble's constantdue to its infrared vision, which sees through dust to provide better views of variable stars called Cepheids. These pulsating stars are vital "rungs" in the distance ladder.

Spitzer observed ten Cepheids in our own Milky Way galaxy and 80 in a nearby neighboring galaxy called the Large Magellanic Cloud. Without the cosmic dust blocking their view at the infrared wavelengths seen by Spitzer, the research team was able to obtain more precise measurements of the stars' apparent brightness, and thus their distances.

With these data, the researchers could then tighten up the rungs on the cosmic distant ladder, better determining distances to other galaxies, and calculate a new and improved estimate of our universe's expansion rate.* The galaxies used in this composite artwork are all infrared images from Spitzer covering wavelengths of 3.6 microns (blue), 4.5 microns (green), and 8.0 microns (red).

Not by accident, the Hubble Constant is named after one of the most fascinating men in the history of science. Born in 1889, ten years after Einstein (of whom he had little knowledge), few greats have had more effect on our knowledge of the cosmos than Edwin Hubble. A naturally-gifted track star, and scholar, the Missouri-born Hubble spent his life following a doctorate in astronomy from the University of Chicago answering two of the most fundamental and profound questions about our universe: how big is it, and how old?

When Hubble moved to California in 1919 to take up a position at the Mount Wilson Observatorynear Los Angles, little was know about the size and age of the universe. The number of known galaxies at the time he first looked out to the cosmos from Mt Wilson was exactly one: the Milky Way. The Milky Way was thought to embrace the entire cosmos with everything else, distant puffs of celestial gas.

Hubble's great breakthrough came in 1923 when with a fresh eye he showed that a distant cloud of that peripheral celestial gas in the Andromeda Constellation known as M31 wasn't a gas cloud, but a maze of brilliant stars, a "nebulae" (Latin for "cloud") -a galaxy a 100,000 light years across and at least 900,000 light years distant from Earth.

This discovery led to his 1924 research paper "Cephids in Spiral Nebulae" (Hubble's term for galaxy) showing that the universe -which we now know houses some 130 billion galaxies- was made up of not just the Milky Way, but a myriad of "island universes," many far more distant and larger.*Hubble then turned to the next question of equally cosmic proportions,just how big is the universe, and made an equally striking discovery: that all the galaxies except for our local cluster are moving away from us at a speed and distance that are nearly proportional. In short, the more distant the galaxy, the faster it was moving.

The concept of an expanding universe destroyed the old, longstanding notion of a static steady-state universe, the wonder of which Stephen Hawking has exclaimed, was that it wasn't obvious before that a static universe would have collapsed in upon itself.

Hubble's ignorance of Einstein's General Theory of Relativity led to his nor being able to connect the dots between a universe that was expanding evenly in all directions (the "Hubble Constant") to a geometrical starting point, a "primeval atom, a Big Bang. That answer came several decades later with the discovery of cosmic background radiation from a hissing, constant, uniform low-frequency radio signal at a Bell Labs facility in rural New Jersey.